iTunes doesn't recognize my computer as authorized anymore

I haven't made a hardware change to this computer since January, and it's always worked fine. I've rented several movies. This was my one authorized computer. This afternoon, the 9.0.1 upgrade ran and upgraded iTunes. I now have 9.0.1.8. Tonight I wanted to rent a movie, and it says my computer isn't authorized! According to the account info page, there is 1 authorized computer associated with my account. It should be this one! Why doesn't iTunes recognize my computer as authorized anymore? I think the iTunes upgrade messed something up, because it worked fine before.

Win XP Pro SP3
iTunes 9.0.1.8
The movie I tried to rent: Fanboys